Carbon fibers are light in weight, yet excellent in strength and elastic modulus, and therefore they are generally combined with various matrix resins to provide composite materials to serve in many fields including aircraft members, spacecraft members, automobile members, ship members, civil engineering construction materials, and sports goods. A typical example of composite material formed mainly of carbon fibers is molded products manufactured by press-molding (a molding method including defoaming under pressure and shaping) a preform produced by stacking prepreg layers. Such a prepreg is generally produced by impregnating a carbon fiber substrate formed mainly of continuous carbon fibers aligned in one direction with resin. Composite materials formed mainly of discontinuous carbon fibers (chopped, web, etc.) that are high in shape conformity to complicated shapes and can be molded in a short time have also been proposed, but prepregs can provide better structural materials in terms of mechanical properties such as specific strength and specific rigidity and the stability of such properties.
In the area of carbon fiber composite materials in recent years, there are stronger demands for molding materials that are high in formability, handleability, and suitability for production of molded products with good mechanical properties, and currently higher economic efficiency and productivity are called for in industrial fields. As one of the solutions to these requirements, studies for the development of prepregs based on thermoplastic resin as matrix resin are in progress.
To take advantage of excellent properties of carbon fibers, it is important to achieve improved adhesion between carbon fibers and matrix resin. To achieve improved interfacial adhesion between carbon fiber bundles and a matrix resin, a generally used method is to subject the carbon fiber bundles to oxidation treatment such as gas phase oxidation and liquid phase oxidation in order to introduce an oxygen-containing functional group on the carbon fiber surface. For example, Patent document 1 proposes a method of subjecting carbon fiber bundles to electrolytic treatment in order to improve the interlaminar shear strength, which is an indicator of the interfacial adhesion strength.
If surface modification of carbon fibers alone does not achieve a sufficient degree of interfacial adhesiveness, an attempt is made to perform additional sizing treatment. For example, Patent document 2 proposes a method of applying polyethyleneimine as a sizing agent to carbon fiber bundles in order to achieve improved adhesion to a thermoplastic resin with few functional groups. In addition, Patent document 3 adopts a method of applying polyethylenimine as a sizing agent after carrying out high-order processing of a carbon fiber bundle to form a web etc. In Patent document 4, furthermore, a high molecular weight, high viscosity polyethyleneimine material is used as a binding agent to add to a carbon fiber bundle in order to produce chopped carbon fibers that will not disperse easily in an injection molding machine.
Patent Documents 5 and 6 each propose a method of using an amine compound and a surfactant as lubricants in order to suppress the fluff formation in the process of producing fibers.
As described above, in the field of composite materials formed of continuous or discontinuous carbon fibers, studies have been made to achieve improvement in adhesion and to realize the suppression of fluff formation using lubricants and the improvement in fiber spreadability. On the other hand, in cases where the above techniques are applied to prepreg containing a thermoplastic resin as a matrix resin, there has been no idea of suppressing the occurrence of impregnation unevenness and voids by achieving both high spreadability of sizing agent-coated carbon fiber bundles and high adhesiveness to the matrix resin in order to provide a high viscosity thermoplastic resin with improved impregnability.